Heretofore, since in a spark plug used for ignition in an internal combustion engine such as an automotive gasoline engine, gasoline and air are mixed and combustion in a short time within a combustion chamber at an engine head, incomplete combustion is liable to be caused to allow carbon or the like to be adhered to a front end portion of an insulator of the spark plug, thus being causative of fouling. When a large amount of carbon is adhered to the front end portion of the insulator of the spark plug, spark is not produced at the front end of the spark plug but a leakage (electric leakage) phenomenon in which spark is produced at a different portion to which carbon is adhered. On the other hand, the carbon has a property of all being burned out when heated up to about 520° C. or more. By noticing such a character of carbon, it is generally used such a spark plug having a self-cleaning function of increasing in the temperature up to about 520° C. at a stretch and burning away carbon by itself.
Further, a spark plug is configured so that a nearly tubular insulator, in which a center electrode that produces spark discharge (spark) is disposed so as to protrude from a front end thereof, is fitted in a nearly tubular metallic shell. The diameter of the front end portion of the insulator is smaller than that of the axially intermediate portion thereof, and the stepped portion between the front end portion and the intermediate portion is adapted to engage the stepped portion provided to the inside of the metallic shell. Heretofore, a large gap between the front end portion of the insulator and the metallic shell has been considered effective for preventing spark jumping produced across the gap at the time of fouling of the insulator. However, if the width of the gap is made larger, the outer diameter of the metallic shell becomes larger and therefore the spark plug itself becomes larger in size. Thus, it has been proposed such a spark plug that is configured to make a gap be equal to or smaller than a predetermined value at a more front end side than an engagement position of the insulator and the metallic shell, thereby being capable of assuredly stopping intrusion of unburnt gas into the gap at the engagement position and preventing fouling of the spark plug (refer to, for example, Patent Document 1).
On the other hand, for the purpose of reducing the fuel consumption of the automotive vehicle, development of lean burn (combustion of a lean air-fuel ratio mixture) in the engine has been made, and when the engine is in such a lean burn condition the spark plug is required to carry out ignition assuredly and therefore it is necessitated to make higher the ignition voltage of the spark plug. Thus, it has also proposed such a spark plug that is configured to provide the inside of the metallic shell with a stepped portion for engagement with the insulator and make the front end side of the insulator, which is positioned opposite to the corner of the stepped portion, be shaped to have the thickness that is equal to or larger than that of the joining portion between the stepped portion and the leg portion of the insulator, whereby the electrical insulation resistance of the insulator leg portion positioned opposite to the corner of the stepped portion of the metallic shell can be improved and production of a pin hole can be inhibited (refer to, for example, Patent Document 2).    Patent Document 1: Unexamined Japanese Patent Publication No. 2002-260817.    Patent Document 2: Unexamined Japanese Patent Publication No. 6-196247.
However, in the spark plug described in the Patent Document 1, the engagement gap between the metallic shell and the insulator is designed to be small, thus being capable of preventing intrusion of unburnt gas but causing the thermal value to become low (low heat radiation rate, removal of heat being bad) since the small gap portion is short in the length in the axial direction of the spark plug, transfer of heat from the insulator to the metallic shell becomes difficult. As a result, the front end portion of the spark plug is excessively heated, thus causing a problem that there is a possibility of causing natural combustion (pre-ignition) before ignition. Further, in the spark plug described in the Patent document 2, the width of the engagement gap between metallic shell and the insulator is adjusted so as to be large, so that while the thermal value is large, intrusion of unburnt gas cannot be prevented, thus causing a problem that fouling is liable to be caused. In this manner, by the technique described in the Patent Documents 1 and 2, if one of the fouling resistance and the heat resistance (thermal value) is improved, the other is deteriorated, and therefore it is difficult to maintain both of the fouling resistance and the heat resistance good.